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IE 419/519
Wireless Networks
Lecture Notes #5
Antennas and Propagation
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Introduction
An antenna is a transducer thatconverts radio frequency electriccurrent to electromagnetic waves
that are radiated into space In two-way communication the
same antenna can !e used for
transmission and reception
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undamental Antenna$oncepts %eciprocity
%adiation Patterns Isotropic %adiator
&ain
Polari'ation
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%eciprocity
In general the various propertiesof an antenna apply equallyregardless of whether it is used for
transmitting or receiving)ransmission*reception e+ciency
&ain
$urrent and voltage distri!ution Impedance
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%adiation Patterns
%adiation pattern &raphical representation of radiation properties of
an antenna
,epicted as a two-dimensional cross section
%eception pattern %eceiving antennas equivalent to radiation pattern
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%adiation Patterns /cont01
eam width /or half-power !eam width1 3easure of directivity of antenna
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Antenna &ain
Antenna gain Power output in a particular direction
compared to that produced in any
direction !y an isotropic antenna 6ective area
%elated to physical si'e and shape of
the antenna
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Antenna &ain /cont01
%elationship !etween antenna gainand e6ective area
G antenna gain
Ae e6ective area f carrier frequency c speed of light /" 8 9:7m*s1 carrier wavelength
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Antenna &ain /cont01
An antenna with a G < "d improvesover the isotropic antenna in thatdirection !y "d or a factor of 2
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Polari'ation
,e=ned as the orientation of theelectric =eld /-plane1 of anelectromagnetic wave
)ypes of polari'ation Linear
>ori'ontal
?ertical $ircular
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Polari'ation
?ertically Polari'ed Antenna lectric =eld is perpendicular to the arths surface
e0g0 roadcast tower for A3 radio @whip antennaon an automo!ile
>ori'ontally Polari'ed Antenna lectric =eld is parallel to the arths surface e0g0 )elevision transmission /B0C01
$ircular Polari'ed Antenna Dave radiates energy in !oth the hori'ontal and
vertical planes and all planes in !etween
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Polari'ation
http://www.phy.ntnu.edu.tw/java/emWave/emWave.htmlhttp://www.phy.ntnu.edu.tw/java/emWave/emWave.html7/23/2019 Lec05 IE419 Antennas 13 Stds
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9"
)ypes of Antennas
Isotropic antenna Ideali'ed %adiates power equally in all directions
Omnidirectional
,ipole antennas >alf-wave dipole antenna
>ert' antenna
Euarter-wave vertical antenna 3arconi antenna
Para!olic %eFective Antenna
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,ipole Antenna
http://www.rfcafe.com/references/electrical/antenna_patterns.htm
Powerradiated
A'imuth
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Propagation 3odes
&round-wave propagation
CGy-wave propagation
Line-of-sight propagation
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&round Dave Propagation
ollows contour of the earth
$an propagate considera!le distances
requencies up to 2 3>'
8ample A3 radio
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CGy Dave Propagation
Cignal reFected from ioni'ed layer ofatmosphere !acG down to earth
Cignal can travel a num!er of hops !acG andforth !etween ionosphere and earths surface
%eFection e6ect caused !y refraction
8amples
Amateur radio
$ radio
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Line-of-Cight Propagation
)ransmitting and receiving antennas must !ewithin line of sight
%efraction ending of microwaves !y the atmosphere ?elocity of electromagnetic wave is a
function of the density of the medium Dhen wave changes medium speed
changes Dave !ends at the !oundary !etween
mediums
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Line-of-Cight quations
Hptical line of sight
6ective /or radio1 line of sight
d< distance !etween antenna and
hori'on /Gm1 h< antenna height /m1 < adJustment factor to account for
refraction rule of thum! < (*"
hd 57.3=
hd = 57.3
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2:
Line-of-Cight quations
3a8imum distance !etween twoantennas for LHC propagationK
h9< height of antenna one
h2< height of antenna two
( )21max 57.3 hhd +=
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LHC Direless )ransmissionImpairments
Attenuation and attenuation distortion
ree space loss
Noise
Atmospheric a!sorption
3ultipath
%efraction
)hermal noise
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Attenuation
Ctrength of signal falls o6 with distanceover transmission medium
Attenuation factors for unguided mediaK %eceived signal must have su+cient strength
so that circuitry in the receiver can interpretthe signal
Cignal must maintain a level su+cientlyhigher than noise to !e received without error
Attenuation is greater at higher frequenciescausing distortion
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2"
ree Cpace Loss
ree space loss Ideal isotropic antenna
Pt< signal power at transmitting antenna Pr< signal power at receiving antenna < carrier wavelength d< propagation distance !etween antennas c< speed of light /" 8 9:7m*s1
where d and are in the same units (e.g.,meters)
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ree Cpace Loss
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ree Cpace Loss
ree space loss accounting for gain ofother antennas
Gt< gain of transmitting antenna
Gr< gain of receiving antenna
At< e6ective area of transmitting antenna
Ar< e6ective area of receiving antenna
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2.
$ategories of Noise
)hermal Noise
Intermodulation noise
$rosstalG Impulse Noise
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)hermal Noise
)hermal noise due to agitation ofelectrons
Present in all electronic devices
and transmission media $annot !e eliminated unction of temperature
Particularly signi=cant for satellitecommunication
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)hermal Noise
Amount of thermal noise to !e found in a!andwidth of 9>' in any device orconductor isK
N:< noise power density in watts per 9 >' of!andwidth
G < olt'manns constant < 90"7:" 9:-2"M*o T < temperature in Gelvins /a!solute
temperature1
( )W/Hzk0 TN =
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)hermal Noise
Noise is assumed to !e independent offrequency
)hermal noise present in a !andwidth of
B>ert' /in watts1K
or in deci!el-watts
TBN k=
BTN log10log10klog10 ++=
BT log10log10dBW6.228 ++=
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Noise )erminology
Intermodulation noise Hccurs if signals with di6erent frequencies
share the same medium
$rosstalG Bnwanted coupling !etween signal paths
http://www.cabletet!"g.comhttp://www.cabletet!"g.com
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Noise )erminology
Impulse noise Irregular pulses or noise spiGes
Chort duration and of relatively highamplitude
$aused !y e8ternal electromagneticdistur!ances or faults and Faws in thecommunications system
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Hther Impairments
Atmospheric a!sorption Dater vapor and o8ygen contri!ute to
attenuation
3ultipath H!stacles reFect signals so that
multiple copies with varying delays are
received %efraction
ending of radio waves as theypropagate through the atmosphere
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ading in 3o!ilenvironment ading
)ime variation of received signal powercaused !y changes in transmissionmedium or path/s1
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3ultipath Propagation /3P1
%eFection Hccurs when signal encounters a surface that
is large relative to the wavelength of the signal
,i6raction Hccurs at the edge of an impenetra!le !ody
that is large compared to wavelength of radiowave
Ccattering Hccurs when incoming signal hits an o!Ject
whose si'e is in the order of the wavelength ofthe signal or less
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)he 6ects of 3PPropagation 3ultiple copies of a signal may arrive at
di6erent phases If phases add destructively the signal level
relative to noise declines maGing detectionmore di+cult
nown as Intersym!ol Interference /ICI1
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)ypes of ading
ast fading
Clow fading
lat fading
Celective fading
%ayleigh fading
%ician fading
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ading